go.tools/dashboard/app: return specific error string on commit not found
go.tools/dashboard/app: return specific error string on commit not found

TBR=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/153070043

d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools